The determination of the flow rate of fluids flowing within a well is important to the monitoring and control of the movement of the fluids in the well and reservoir. For example, by monitoring the flow rates of both oil and water from each zone of a well, the water production of the entire well may be controlled by reducing the flow from those zones that produce the highest water cut (i.e., ratio of water flow rate to total flow rate), allowing the reservoir oil to be swept more completely during the life of the well.
One common method for determining the velocity of a fluid in a flow stream involves disposing a turbine blade within the flow stream and measuring the rotational velocity of the turbine blade. In single phase flow conditions, the rotational velocity of the turbine blade is simply related to the velocity of the flow stream. Unfortunately however, in multiple phase flow conditions, such as in a mixed oil and water flow condition, the response of the turbine can be so complicated that the results may not be interpretable. Another method for determining the velocity of a fluid in a flow stream involves injecting a tracer substance into the fluid phase of choice (oil or water) and measuring the time it takes for the tracer substance to travel a known distance in the flow stream. The velocity may then be computed using the known distance and the time of travel. One disadvantage of this method for permanent down-hole use is the need for a reservoir of tracer material and a mechanical tracer injector. Because the reservoir and injector are permanently disposed down-hole in the well, the number of velocity measurements is limited by the quantity of tracer material and the injector is prone to sticking and failure.
Another method for determining the velocity of a fluid in a flow stream involves using local capacitance or resistance sensors. However, this method is only appropriate for flow regimes in which one phase is dispersed as droplets in another continuous phase. As a droplet passes one of the sensors, a signal is produced for a time duration related to the speed of the droplet. Given knowledge of the droplet size by other means, the velocity of the droplet, and hence the fluid flow, can be deduced. One disadvantage of this method is that it does not work at all in a stratified flow regime because it relies on the existence of bubbles.
Another method for determining the flow rate of a fluid in a flow stream involves using a Venturi to measure the total volumetric flow rate. Unfortunately however, due to insensitivity to errors in density and/or pressure determinations, this method may be inaccurate and thus, unreliable.
As such it is an objective of the invention to provide a method for determining the true flow rate of a fluid flowing in a flow line of a well. It is another objective of the invention to provide a method for identifying the bubble point of a fluid flowing in a flow line of a well.